A wireless communication (e.g. cellular, PCS, and so forth) system generally consists of three components, a mobile telephone switching office (MTSO), a number of cell sites, and a number of radiotelephones. Typically, a radiotelephone communicates over a radiofrequency link to a base transceiver station portion of a cell site. The cell site provides the interface between the MTSO and the radiotelephone and interconnects with the MTSO through a high-speed digital transmission link. This high-speed digital transmission link is configured to carry both voice conversations and data, as well as cell site control information between the MTSO and the cell site.
The MTSO is the coordinating element for the cell sites in the cellular system. The MTSO conventionally contains a processor and a cellular switch for connecting cellular subscribers to other cellular subscribers and to external networks, such as the public switched telephone network (PSTN). The MTSO is interconnected to a wireline central office of the PSTN using high-speed digital transmission links configured to carry voice conversations.
The high-speed digital transmission links between each of the cell sites and the MTSO are conventionally microwave links or PSTN-provided T-1 wirelines. The high-speed digital transmission links between the MTSO and the wireline central office are conventionally PSTN-provided T-1 wirelines. The conventional T-1 wireline has a capacity of 1.544 Mbps and accommodates twenty-four digital service, level 0 (DS-0) channels. Each DS-0 channel can transmit at a rate of 64 kbps, which is the worldwide standard speed for digitizing one voice conversation using pulse code modulation (PCM).
Typically, a cell site will not utilize all twenty-four DS-0 channels of the T-1 wireline which interconnects the cell site to the MTSO. More likely, the cell site will utilize fourteen or less DS-0 channels while the remaining DS-0 channels are unused. Thus, the unused DS-0 channels represent an inefficient use of the leased PSTN T-1 wireline. This inefficient usage undesirably drives up cellular system costs.
Another problem with this cellular system interconnection configuration is that when transmission over a T-1 wireline between the MTSO and a cell site is interrupted, the cell site becomes nonoperational for interfacing wireless calls to the PSTN and other cell sites. In other words, subsequent calls between that cell site and the PSTN or other cell sites cannot be completed. The cell site becomes nonoperational because a transmission interruption results in a loss of communication of the cell site control information between the cell site and the MTSO. Transmission over the T-1 wireline can become interrupted for a number of reasons, such as, a break in the T-1 wireline, intermittent wireline failures, unexpected rearrangement of the T1-carrier route by the local telephone company in the PSTN which is serving the area in which the cell site is located, and so forth.
Frequent interruptions to the wireless service can lead to subscriber dissatisfaction and subscriber churn, or cancellation of service. Furthermore, the cell site remains nonoperational until the local telephone company, otherwise known as the local exchange carrier (LEC), responsible for the failed T-1 wireline, repairs the failed T-1 wireline. Thus, wireless calls, and including the especially critical emergency 911 wireless calls, cannot be supported by the non-operational cell site. In other words, in the conventional cellular system architecture, there is no redundant system for ensuring that the cell site remains operational so that wireless calls, including emergency 911 calls, can be delivered.
In addition, without a redundant system for routing signals in the cellular system, cellular providers are compelled to schedule maintenance or repairs of intermittent problems in the T-1 wireline during low usage periods, such as in the middle of the night. LEC personnel costs to provide this maintenance or perform the repairs on the T-1 wireline is excessively costly due to the off-normal working hours.
To circumvent this problem, some cellular providers lease additional PSTN-provided T-1 wirelines in order to provide system redundancy between the cell sites and the MTSO. Thus, when transmission over one of the T-1 wirelines is interrupted, the redundant T-1 wireline can provide alternate routing for cell site control information and for subsequent wireless calls. Unfortunately, additional T-1 wirelines also drive up cellular system costs while leading to even more inefficient usage of T-1 wireline capacity.
Wireless calls can generally be divided into two categories, intra-local exchange calls and extra-local exchange calls. Intra-local exchange calls are those calls for which a local exchange carrier (LEC) end office is a terminating office for the call. In other words, wireless intra-local exchange calls are directed to a telephone subscriber whose telephone equipment is connected to the same LEC end office. In contrast, extra-local exchange calls are those calls in which another central office is the terminating office for the wireless call.
FIG. 1 shows a block diagram of an environment 20 in which a prior art cellular system 22 is interconnected with the public switched telecommunication network (PSTN). Cellular system 20 includes a mobile telephone switching office (MTSO) 24, a first cell site 26 and a second cell site 28. A first PSTN-provided T-1 transmission link 30 interconnects MTSO 24 and first cell site 26. A second PSTN-provided T-1 transmission link 32 interconnects MTSO 24 and second cell site 28. A PSTN-provided T-1 wireline 34 interconnects MTSO 24 to a local exchange carrier (LEC) tandem office 36. LEC tandem office 36 is a central office for the local exchange carrier in the PSTN which serves to connect other LEC offices (not shown) for the delivery of calls.
In this prior art configuration, T-1 wireline 34, otherwise known as a Type IIA link, is configured to convey both intralocal exchange and extra-local exchange calls to LEC tandem office 36. A PSTN-provided T-1 wireline 38 interconnects LEC tandem office 36 to a local exchange carrier (LEC) end office 40 having a service area 42. T-1 wireline 38 is also a Type IIA link and is configured to convey intra-local exchange calls to LEC end office 36.
A communication path for an exemplary wireless intra-local exchange call, originated at a radiotelephone 44, is through a radiofrequency link 46, to second cell site 28. The intra-local exchange call is subsequently routed through PSTN T-1 transmission link 32 to MTSO 24, then through Type IIA T-1 wireline 34 to LEC tandem office 36. LEC tandem office 36 switches the intra-local exchange call to Type IIA T-1 wireline 38, and the call is routed over Type IIA T-1 wireline 38 to LEC end office 40. LEC end office 40 subsequently routes the intra-local exchange call over a telephone link 48 to a telephone 50.
The prior art interconnection configuration illustrated in FIG. 1 results in the eventual delivery of the intra-local exchange call. Unfortunately, this intra-local exchange call does not use the PSTN efficiently and therefore incurs unnecessary and costly mileage tariffs due to its routing through LEC tandem office 36 via T-1 wirelines 34 and 38.
In order to circumvent the costly mileage tariffs, some prior art systems lease a PSTN-provided T-1 wireline between MTSO 24 and LEC end office 40, as illustrated by a dashed line 52. T-1 wireline 52, known as a Type IIB link then replaces Type IIA wireline 38 for the routing of intra-local exchange calls. This arrangement improves PSTN usage efficiency, and the lease cost of Type IIB T-1 wireline 52 is essentially the same as the lease cost of Type IIA T-1 wireline 38. However, the mileage tariff per call for Type IIB wireline 52 is zero which is significantly lower than the mileage tariff per call for Type IIA wireline 38. Unfortunately, only a slight improvement in PSTN usage efficiency results, and Type IIB T-1 wireline 52 is not cost effective if the average number of intra-local exchange calls at any given time is not great enough to support the lease cost of Type IIB T-1 wireline 52.
Thus, a need remains for a wireless communication system that provides selectable signal routing for supplying cell sites with cell site operational information and that more efficiently utilizes the PSTN-provided T-1 communication links.